17.03.2021 Better than Tycho Brahe's star catalog

Wilhelm IV, son of the founder of our university, Philipp, Landgraf von Hessen, initiated and organized new measurements of stellar positions. The result: the Hessian catalog surpassed Brahe's catalog by a factor of two in accuracy.

The manuscript of the star catalogue of Wilhelm IV is in the collection of the University of Kassel, available on ORKA, with shelf mark 2◦ Ms. astron. 7., https:\\orka.bibliothek.uni-kassel.de/viewer/image/1336543085355/1/

Near the end of the 16th century Wilhelm IV, Landgraf von Hessen-Kassel, set up an observatory with the main goal to increase the accuracy of stellar positions primarily for use in astrology and for calendar purposes.

Wilhelm IV was first to alert the European astronomical world to the need of new observations for more accurate positions of the stars and – since the positions of planets were measured with respect to nearby stars – of the planets, and to act on it!

A new star catalog was compiled from measurements of altitudes and angles between stars. The measurements and the computations are very accurate, thanks to the instrument maker and mathematician Jost Bürgi. The superior clocks of Jost Bürgi made it possible to use time differences instead of angles, thus introducing a new technique for determining distances of stars and paving the way to transit measurements.

A print ready version was prepared listing measurements as well as equatorial and ecliptic coordinates of stellar positions. Unfortunately, this catalog appeared in print not before 1666, long after the dissemination of Brahe’s catalogue. With the data given in the manuscript we are able to analyze the accuracy of measurements and computations. The positional errors corrected for an offset of 6' in the right aszension α is 1.4' ± 1.0'. The star catalogue is more accurate by a factor two than the later catalog of Tycho Brahe.

Positional error distribution, corrected for the 6’ offset in α, for the 382 entries in Manuscript, after excluding two star clusters and three repeated entries, for all magnitudes, and for each magnitude separately. The numbers in each frame indicate the number of entries with errors smaller / larger than the frame limit of 8′.

Positional error distribution, corrected for the 6’ offset in α, for the 382 entries in Manuscript, after excluding two star clusters and three repeated entries, for all magnitudes, and for each magnitude separately. The numbers in each frame indicate the number of entries with errors smaller / larger than the frame limit of 8′.

Original publications

• "The star catalogue of Wilhelm IV, Landgraf von Hessen-Kassel. Accuracy of the catalogue and of the measurements"
  Frank Verbunt, Andreas Schrimpf, Astronomy & Astrophysics, in press (2021),
  DOI: https://doi.org/10.1051/0004-6361/202140332, preprint, https://arxiv.org/abs/2103.03034
• "The star catalogue of Wilhelm IV, Landgraf von Hessen-Kassel."
  Andreas Schrimpf, Frank Verbunt, in "Applied and computational historical astronomy", ed. G. Wolfschmidt, Nuncius Hamburgensis, Beiträge zur Geschichte der Naturwissenschaften, in press, (2021),
  https://arxiv.org/abs/2103.10801

Media response

• Der Kopernikus aus Kassel, Der Spiegel 20/2021
• Wilhelm IV, the obscure German princeling who put stars on the map, The Times, Mo 17 May 2021